The present invention concerns redispersible polymer powders, which excel especially by good storage stability and can be used in many ways, for example, in hydraulically binding systems, but also as a binder in the most varied applications, and can be prepared by spray-drying of aqueous dispersions.
The use of polymer powders as additive, especially for hydraulically binding systems or as binder in coating compositions is generally known. Such additives improve the application properties of the systems in which they are used. Thus, when used in hydraulically binding compositions, the bending and tensile strength of construction materials to which they are added, are improved. Moreover, they have an improving influence on the processability of the construction materials.
The production of such powders is known and, for example, is described in the following patents:
In EP 0134451, the use of various naturally occurring water-soluble substances, such as starches, starch derivatives, proteins and/or water-soluble polymers are named as spray additives in the preparation of redispersible dispersion powders.
DE-OS 4021216 describes the preparation of a polymer powder, where a film-forming dispersion polymer is sprayed with an completely or partially neutralized water-soluble copolymer consisting of 50 to 80 weight % of unsaturated carboxylic acids and an alkene and/or styrene as spray additive. According to EP 0 671 435, aqueous protective-colloid-containing acrylate dispersions can be sprayed to powder redispersible binders using 0.01 to 30 weight % of water-soluble or water-dispersible resins, which consist of 30 to 50 weight % of hydrophilic, water-soluble and salt-forming monomers and 50-70 weight % of hydrophobic, essentially water-insoluble monomers. In EP 0522791, the preparation of powders by spraying of core-shell products is described, where an alkali-insoluble core is surrounded by an alkali-soluble shell. The core-shell dispersion is sprayed after neutralization with an amine or a base.
All the redispersible powders named above are based on a uniform basic principle of preparation: To the polymer to be sprayed, a spraying aid is added, which is first in the dissolved form at the beginning of the spray-drying process and deposits on the particles to be sprayed during drying, as a xe2x80x9cprotective shieldxe2x80x9d. The spraying aid has a great influence on the properties of the powder. First of all, it should provide as good a redispersibility to the polymer powder as possible, and, on the other hand, together with the commonly used antiblocking agent, should prevent blocking of the powder during storage. Moreover, the spraying aid should not have an adverse influence on the application-technological properties of the polymer to be sprayed, but should influence these as advantageously as possible.
The task of the present invention was to prepare a redispersible polymer powder which is characterized, in contrast to the above, by the fact that, with the same amount of spraying aid (weight %, based on the polymer to be sprayed), a clearly improved resistance to blocking is achieved. Moreover, the preparation of the powder according to the invention should result in a simplification of the manufacturing technology.
In developing the solution, it was shown that good redispersible dispersion powders can be prepared not only in the usual way, by the use of a spraying aid present in the dissolved form, but that, surprisingly, this is possible even when using an additive that is not dissolved, when this has a particle size which is clearly smaller than the particle size of the dispersion to be sprayed.
This is achieved by redispersible polymer powders, prepared by spray-drying of an aqueous dispersion, which is obtained by mixing an aqueous polymer dispersion 1 (containing polymer I) with an aqueous polymer dispersion 2 (containing polymer II), where the diameter of the particles of polymer I is greater than the diameter of the particles of polymer H by a factor of 5 to 150, preferably 10 to 50, and polymer I is a polymer that forms films at room temperature, while polymer II has a glass-transition temperature which is higher than that of polymer I, is insoluble in water and thus it is present in the undissolved form in the aqueous dispersion to be sprayed.
The glass temperature of polymer H should be higher than the temperature at the exit of the spray dryer. The amount of polymer II, which is needed (weight % based on polymer I), in order to achieve a certain resistance to blocking, which becomes smaller as polymer I and polymer II differ more with regard to their particle size. The amount of polymer II which is used lies at 5 to 40 weight %, preferably 10 to 30 weight %. Before spraying the additive, surface-active substances can be added to the aqueous dispersion to improve the redispersibility.
Preferably, polymer II consists of 5 to 80 weight %, especially 20 to 50 weight % of a copolymerized, unsaturated carboxylic acid and is thus entirely or partially soluble in alkali, while the pH value of the aqueous dispersion to be sprayed is kept so low that polymer II is present in the undissolved form.
Advantageously, methacrylic acid or acrylic acid is used as the unsaturated carboxylic acid.
The spray drying of the aqueous dispersion is done in a commercial pilot plant spray dryer in cocurrent. Air is used as drying gas, which enters the dryer at a temperature of 115EC and leaves it at a temperature of 60EC. The dispersions I and II are always sprayed in as a mixture of dispersion through a two-material nozzle. The addition of antiblocking agents (ABM) is done through a second two-material nozzle also in cocurrent. The powder is deposited in a cyclone which is connected at the end.